The present invention relates to a gas-feeding device for use in the production of a molded article having an excellent appearance free from sink marks and distortion, by an injection molding method. More specifically, it relates to a gas-feeding device for feeding a pressurized gas into a molten resin injected into a cavity of a mold, attached to an injection molding machine for producing a molded article having a hollow portion.
For example, JP-A-64-14012 (corresponding to U.S. Pat. No. 4,740,150) discloses an injection molding machine used for injection molding a molten thermoplastic resin for producing a molded article having an excellent appearance free from sink marks and distortion. In the injection molding method disclosed in JP-A-64-14012, as shown in FIG. 1 attached thereto, a molten plastic material 19 is injected into a mold cavity 13, then a pressurized gas is fed into the molten plastic material 19 in the mold cavity 13 to form a gas-containing hollow portion 25 within the plastic material 19, and the gas within the gas-containing hollow portion 25 is released into atmosphere before the mold is opened.
In the gas-feeding device disclosed in the above JP-A-64-14012, for feeding the pressurized gas into the molten plastic material 19 in the mold cavity 13, a nozzle 26 is shifted to a forward position as shown in FIG. 2 attached to JP-A-64-14012. In this state, a cap 32 of the nozzle 26 is in contact, under pressure, with a conical valve seat of a valve opening 42 provided to an insert 41 in a lower mold member 12. The contact of the cap 32 of the nozzle 26 and the valve seat of the valve opening 42 under pressure can prevent the molten plastic material 19 injected into the mold cavity 13 from leaking around the nozzle 26. Further, the pressurized gas can be reliably fed into the molten plastic material 19. When the pressurized gas in the gas-containing hollow portion 25 is released to atmosphere, the nozzle 26 is shifted to a backward position as shown in FIG. 3 attached to JP-A-64-14012. In this state, the cap 32 of the nozzle 26 is apart from the valve opening 42, and the gas in the gas-containing hollow portion 25 is released through the valve opening 42 around the valve 26 and further through a gap formed by the nozzle 26 and the insert 41 to atmosphere. The nozzle 26 is shifted under the control of a piston 29 and a cylinder 27.
In an injection molding machine having the above nozzle 26 and valve opening 42, however, the centerline of the nozzle 26 and the centerline of the valve opening 42 may be sometimes out of line, or a so-called centerline deviation may occur. This centerline deviation sometimes occurs at a mold manufacturing time or an assembly time, or the movements of the piston 29 and the cylinder 27 which put the nozzle 26 in forward and backward movement cause the centerline deviation in some cases.
When the above centerline deviation occurs, a gap is formed between the nozzle 26 and the valve seat of the valve opening 42, and the pressurized gas is insufficiently sealed when it is fed into the molten plastic material 19 injected into the mold cavity 13. As a result, part of the pressurized gas leaks through this gap. Therefore, the pressurized gas can be no longer sufficiently fed into the plastic material 19, and the gas-containing hollow portion 25 is hardly formed within an injection molded article. Otherwise, the gas-containing hollow portion 25 which may be formed no longer has a desired size (volume). As a consequence, the molded article has a sink mark and distortion, and has a poor appearance. Further, when a large centerline deviation occurs, there is another problem in that the plastic material 19 injected into the mold cavity 13 leaks through the gap formed between the nozzle 26 and the valve seat of the valve opening 42 to adhere to an outer wall of the nozzle 26 or to leak out of the mold. When the plastic material 19 adheres to the outer wall of the nozzle 26, the nozzle 26 may be no longer shifted.
The failure in the contact between the nozzle 26 and the valve seat of the valve opening 42 under pressure is caused not only by the centerline deviation between the nozzle 26 and the conical valve seat of the valve opening 42, but also by any one of the defective finish of surface processing of the valve seat of the valve opening 42 and the adhesion of a resin scrap, mold abrasion dust or other foreign substance to the valve seat surface of the valve opening 42. In any one of the above cases, when the nozzle 26 is shifted to a forward position, a gap is formed between the nozzle 26 and the valve seat of the valve opening 42. Further, when a molten thermoplastic resin is injected into the mold cavity 13 without shifting the nozzle 26 to a forward position because of an operation failure or for some other reason, the nozzle 26 and the valve seat of the valve opening 42 are not in a contact state under pressure, and the above problem also takes place.